1. Field of the Invention
The present invention generally relates to a shutter apparatus for camera, and particularly relates to a shutter apparatus for camera of a type in which piezoelectric element is employed as driving means for opening and closing the shutter blades.
2. Description of the Prior Art
Conventionally, the shutter apparatus of the above type includes a single driving lever for opening and closing the shutter blades, which lever consists of a piezoelectric element. One end of the lever is supported by a support means provided in a camera, on one hand, while the other end, i.e. the free end, can be curved or moved in the opposite directions to drive the shutter blades.
Incidentally, the shutter apparatus of a camera, generally, requires that the shutter blades can be closed momentarily after the passage of a predetermined exposure time. In addition, the time period to be consumed for closing the shutter blades should be as shorter as possible.
It should be noted that the conventional shutter apparatus has such a shutter characteristic as shown in FIG. 8. The shutter characteristic is defined by a voltage-movement characteristic of the piezoelectric element which is shown in FIG. 8. In FIG. 8, the abscissa axis represents the time, while the ordinate axis represents the stroke or amount of movement of the element, and the solid curved line represents a wave-form of movement of the element. As apparent from FIG. 8, since the element carries the inertial system, i.e. the shutter blades and etc., delay of response to the shutter closing signal occurs, resulting in that a long time period t1 is consumed for fully closing the shutter blades. As previously described, the time period t1 should be as shorter as possible. If the time period t1 is quite long, the high speed shutter apparatus can not be obtained.